High dose implantation strip (HDIS) in H2 base chemistry

ABSTRACT

Plasma is generated using elemental hydrogen, a weak oxidizing agent, and a fluorine containing gas. An inert gas is introduced to the plasma downstream of the plasma source and upstream of a showerhead that directs gas mixture into the reaction chamber where the mixture reacts with the high-dose implant resist. The process removes both the crust and bulk resist layers at a high strip rate, and leaves the work piece surface substantially residue free with low silicon loss.

RELATED U.S. APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 11/712,253, filed Feb. 27, 2007, titled“Enhanced Stripping of Low-K Films Using Downstream Gas Mixing,” whichis a divisional of U.S. patent application Ser. No. 11/011,273, issuedas U.S. Pat. No. 7,202,176, filed Dec. 13, 2004, and titled “EnhancedStripping of Low-K Films Using Downstream Gas Mixing,” the disclosuresof these are incorporated herein by reference in their entireties andfor all purposes. The present application claims priority to and benefitof each of these applications, as provided under 35 U.S.C. 120.

FIELD OF INVENTION

The present invention pertains to methods and apparatuses to remove orstrip photoresist material and removing related residues from a workpiece surface. Particularly, this application relates to methods andapparatus for stripping resist after ion implant or plasma assistingdoping implant (low dose or high-dose implanted resist).

BACKGROUND

Photoresist is a light sensitive material used in certain fabricationprocesses to form a patterned coating on a work piece, e.g., asemiconductor wafer, during processing. After exposing the photoresistcoated surface to a pattern of high energy radiation, a portion of thephotoresist is removed to reveal the surface below, leaving the rest ofthe surface protected. Semiconductor processes such as etching,depositing, and ion implanting are performed on the uncovered surfaceand the remaining photoresist. After performing one or moresemiconductor processes, the remaining photoresist is removed in a stripoperation.

During ion implantation, dopant ions, e.g., ions of boron, borondifluoride, indium, gallium, thallium, phosphorous, arsenic, antimony,bismuth, or germanium, are accelerated toward a work piece target. Theions implant in exposed regions of the work piece as well as in theremaining photoresist surface. The process may form well regions(source/drain) and lightly doped drain (LDD) and doubled diffused drain(DDD) regions. The ion implant impregnates the resist with the implantspecies and depletes the surface of hydrogen. The outer layer or crustof the resist forms a carbonized layer that may be much denser than theunderlying bulk resist layer. These two layers have different thermalexpansion rates and react to stripping processes at different rates.

The difference between the outer layer and bulk layer is quitepronounced in post high-dose ion implant resist. In high-doseimplantation, the ion dose may be greater than 1×10¹⁵ ions/cm and theenergy may be from 10 Kev to greater than 100 keV. Traditional high doseimplantation strip (HDIS) processes employ oxygen chemistries wheremonatomic oxygen plasma is formed away from the process chamber and thendirected at the work piece surface. The reactive oxygen combines withthe photoresist to form gaseous by-products which is removed with avacuum pump. For HDIS, additional gases are needed to remove theimplanted dopants with oxygen.

Primary HDIS considerations include strip rate, amount of residue, andfilm loss of the exposed and underlying film layer. Residues arecommonly found on the substrate surface after HDIS and stripping. Theymay result from sputtering during the high-energy implant, incompleteremoval of crust, and/or oxidation of implant atoms in the resist. Afterstripping, the surface should be residue free or substantially residuefree to ensure high yield and eliminate the need for additional residueremoval processing. Residues may be removed by overstripping, i.e., acontinuation of the strip process past the point nominally required toremove all photoresist. Unfortunately, in conventional HDIS operations,overstripping sometimes removes some of the underlying functional devicestructure. At the device layer, even very little silicon loss from thetransistor source/drain regions may adversely affect device performanceand yield, especially for ultra shallow junction devices fabricated atthe <32 nm design rule or below.

What is needed therefore are improved and methods and apparatus forstripping photoresist and ion implant related residues, especially forHDIS, which minimizes silicon loss and leaves little or no residue whilemaintaining an acceptable strip rate.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned need by providingimproved methods and an apparatus for stripping photoresist and removingion implant related residues from a work piece surface. Plasma isgenerated using elemental hydrogen, a weak oxidizing agent, and afluorine containing gas. In certain embodiments, an inert gas isintroduced to the plasma downstream of the plasma source and upstream ofa showerhead, which directs gas into the reaction chamber. Theplasma-activated gases flowing together with the inert gas reacts withthe high-dose implant resist, removing both the crust and bulk resistlayers, leaving the work piece surface substantially residue free withlow silicon loss.

In one aspect of the invention, methods involve removing material from awork piece in a process chamber according to the following operations:introducing a gas comprising elemental hydrogen, a weak oxidizing agent,and a fluorine containing gas into a plasma source, generating a plasmafrom the gas introduced into the plasma source, and introducing an inertgas downstream of the plasma source and upstream of the work piece. Theplasma-activated gas travels toward a work piece, and is combined withthe inert gas upstream of a showerhead in the reaction chamber. Theelectrically charged species in the plasma may be discharged orpartially discharged when they contact the showerhead.

The plasma-activated gas comprising elemental hydrogen, the weakoxidizing agent, and the fluorine containing gas flows, together withthe inert gas, to the work piece and reacts with the material from thework piece. Examples of weak oxidizing agents include carbon dioxide,carbon monoxide, nitrogen dioxide, nitrogen oxide, water, hydrogenperoxide, or combinations of these. The weak oxidizing agent ispreferably carbon dioxide. The fluorine containing gas may be carbontetrafluoride, other fluorocarbons including hydrofluorocarbons,elemental fluorine, nitrogen trifluoride, sulfur hexafluoride,combinations of these, and the like. The fluorine containing gas ispreferably carbon tetrafluoride. The inert gas may be argon, helium,nitrogen, combinations of these, and the like. The preferred inert gasis argon. The gas introduced into the plasma source may be premixed ornot, and may include about 1% to 99%, or about 0.1% to about 10% orabout 3% to 5% by volume of the weak oxidizing agent. The inert gas maybe introduced at a volumetric flow rate of about 0.15 and 10 times, orabout 2 times, the volumetric flow rate of the elemental hydrogen. Atthe work piece, the gas may include at most about 1% by volume of theweak oxidizing agent species and about 0.1 to 0.5% by volume of thefluorine containing gas species.

In certain embodiments, the material removed from the work piece surfaceis a high-dose implant resist. The work piece may be a 300 mm wafer. Theplasma may be remotely generated using RF power between about 300 wattsand about 10 kilo-watts. The temperature of the work piece may be about160 to 500 degrees Celsius when contacted by the gas. The processpressure may be between about 300 mTorr and 2 Torr.

According to various embodiments, the a high-dose implant resist isremoved from the work piece surface at a rate that is at least about 100nm/min and silicon is removed from the work piece surface at an overallrate of no greater than about 4 nm/min. The resulting work piece issubstantially residue free of the high-dose implanted resist afterremoval, and less than about 3 angstroms silicon is lost from anunderlying silicon layer.

Another aspect of this invention relates to multi-step methods ofremoving high-dose implanted resist from a work piece surface in areaction chamber. The methods include removing a first portion of thematerial by introducing, at a first total flow rate, a first gascomprising elemental hydrogen, a weak oxidizing agent, and with orwithout a fluorine containing gas into a plasma source, generating afirst plasma from the first gas introduced into the plasma source,introducing a first inert gas downstream of the plasma source andupstream of the work piece, and reacting a first portion of the materialfrom the work piece with the mixture. The methods also include removinga second portion of the material by introducing, at a second total flowrate, a second gas comprising hydrogen and a weak oxidizing agent andwith or without a fluorine containing gas into a plasma source,generating a second plasma from the second gas introduced into theplasma source, introducing a second inert gas downstream of the plasmasource and upstream of the work piece, and reacting a second portion ofthe material from the work piece. The first and second gas compositionsare different. In certain embodiments, at least one of the first or thesecond gas includes a fluorine containing gas. At the end of the removalprocess, in certain embodiments, the work piece is substantially residuefree and has had less than about 3 angstroms of silicon lost from anunderlying silicon layer. The removing a second portion operation mayoccur before removing the first portion operation. In certainembodiments, the one or more of the removing operations are repeated oneor more times. These removing operations may occur in a same ordifferent reaction stations in the reaction chamber.

In yet another aspect, the present invention pertains to an apparatusfor removing material from a work piece surface comprising a reactionchamber and a controller. The reaction chamber includes a plasma source,a gas inlet for introducing a gas mixture comprising elemental hydrogeninto the plasma source, a gas inlet for introducing an inert gasdownstream of the plasma source and upstream of the work piece, ashowerhead positioned downstream of the gas inlet, and a work piecesupport downstream of the showerhead. The work piece support includes apedestal and temperature-controlling mechanism to control a temperatureof a work piece supported on the work piece support. The controller isconfigured to execute a set of instructions, including instructions tointroduce a gas comprising hydrogen, a weak oxidizing agent, and afluorine containing gas into a plasma source, generate a plasma from thegas introduced into the plasma source, introduce an inert gas downstreamof the plasma source and upstream of the work piece, and optionally torepeat the introduce a gas, generate a plasma, and introduce an inertgas instructions using different flow rates and gas compositions. Theplasma source used in accordance with the methods and apparatus of theinvention may be any of a number of conventional plasma sources. Forexample, an RF ICP source may be used.

The process chamber used in accordance with the methods and apparatus ofthe invention may be any suitable process chamber. The process chambermay be one chamber of a multi-chambered apparatus or it may be part of asingle chamber apparatus. In certain embodiments, the reaction chamberincludes a plurality of stations, where at least one station includes aplasma source, a plurality of gas inlets, a showerhead, and a work piecesupport.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing an apparatus according tocertain embodiments of the claimed invention and suitable for practicingthe methods of the claimed invention.

FIG. 2A to 2D depicts various stages of semiconductor fabrication beforeand after ion implantation and stripping operations.

FIG. 3 is a process flow diagram showing various operations inaccordance with certain embodiments of the present invention.

FIGS. 4A to 4D depicts before and after strip SEM photos of photoresistpattern stripped under various conditions in accordance with variousembodiments of the present invention.

FIG. 5A is a plot of silicon loss for HDIS using various carbon dioxideflow rates in accordance with various embodiments of the presentinvention.

FIG. 5B is a plot of silicon loss for HDIS using various carbontetrafluoride flow rates in accordance with various embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Introduction

In the following detailed description of the present invention, numerousspecific embodiments are set forth in order to provide a thoroughunderstanding of the invention. However, as will be apparent to thoseskilled in the art, the present invention may be practiced without thesespecific details or by using alternate elements or processes. In otherinstances well-known processes, procedures and components have not beendescribed in detail so as not to unnecessarily obscure aspects of thepresent invention.

In this application, the terms “work piece”, “semiconductor wafer”,“wafer” and “partially fabricated integrated circuit” will be usedinterchangeably. One skilled in the art would understand that the term“partially fabricated integrated circuit” can refer to a silicon waferduring any of many stages of integrated circuit fabrication thereon. Thefollowing detailed description assumes the invention is implemented on awafer. However, the invention is not so limited. The work piece may beof various shapes, sizes, and materials. In addition to semiconductorwafers, other work pieces that may take advantage of this inventioninclude various articles such as displays, printed circuit boards, andthe like.

As mentioned previously, the methods and apparatus of the invention maybe used to efficiently and effectively to remove photoresist materialsafter high-dose ion implantation. The invention is not limited tohigh-dose implant strip (HDIS). The invention is also not limited to anyparticular category of dopants implanted. For instance, describedmethods and apparatus may be effectively used with stripping aftermedium or low dose implant. Although specific dopant ions such as boron,arsenic, and phosphorous are discussed, the described methods andapparatus may be effectively used to strip resist impregnated with otherdopants, such as nitrogen, oxygen, carbon, germanium, and aluminum.

The methods and apparatus of the present invention use plasmas that areproduced from gases that contain hydrogen. The gases also contain a weakoxidizing agent and a fluorine containing gas. One skilled in the artwill recognize that the actual species present in the plasma may be amixture of different ions, radicals, and molecules derived from thehydrogen, weak oxidizing agent, and fluorine containing gas. It is notedthat other species may be present in the reaction chamber, such as smallhydrocarbons, carbon dioxide, water vapor and other volatile componentsas the plasma reacts with and breaks down the organic photoresist andother residues. One of skill in the art will also recognize that theinitial gas/gases introduced into the plasma is/are often different fromthe gas/gases that exist in the plasma as well as the gas/gases contactthe work piece surface during strip.

FIG. 1 is a schematic illustration of an apparatus 100 according tocertain embodiments of the claimed invention. Apparatus 100 has a plasmasource 101 and a process chamber 103 separated by a showerhead assembly105. Plasma source 101 is connected to gas inlet 111. Showerhead 109forms the bottom of showerhead assembly 105. Inert gas inlets 113 aredownstream of plasma source 101 and upstream of wafer 123 and showerhead109. Inside process chamber 103, a wafer 123 with photoresist/dry etchbyproduct material rests on a platen (or stage) 117. Platen 117 may befitted with a temperature control mechanism that may heat or cool awafer on the platen as necessary. In some embodiments, platen 117 isalso configured for applying a bias to wafer 123. Low pressure isattained in reaction chamber 103 via vacuum pump and conduit 119.

In operation, a gas is introduced via gas inlet 111 to the plasma source101. The gas introduced to the plasma source contains the chemicallyactive species that will be ionized in the plasma source to form aplasma. Gas inlet 111 may be any type of gas inlet and may includemultiple ports or jets. Plasma source 101 is where the active species ofthe gas introduced to the source are generated to form a plasma. In FIG.1, an RF plasma source is shown with induction coils 115, which areenergized to form the plasma. An inert gas is introduced via gas inlets113 upstream of the showerhead and downstream of the plasma source. Theinert gas mixes with the plasma. Gas inlets 113 may be any type of gasinlets and may include multiple ports or jets to optimize mixing theinert gas with the plasma. Showerhead 109 directs the plasma/inert gasmixture into process chamber 103 through showerhead holes 121. There maybe any number and arrangement of showerhead holes 121 to maximizeuniformity of the plasma/gas mixture in process chamber 103. Showerheadassembly 105, which may be electrically grounded or have an appliedvoltage, may capture and discharge some ions and thereby change thecomposition of the gas flowing into process chamber 103: i.e., the gaswill contain an increased proportion of neutral species. As mentioned,wafer 123 may be temperature controlled and/or a RF bias may be applied.The plasma/inert gas mixture removes the photoresist/etch byproductmaterial from the wafer.

In some embodiments of the claimed invention, the apparatus does notinclude showerhead assembly 105 and showerhead 109. In theseembodiments, the inert gas inlets 113 introduce the inert gas directlyinto the process chamber where it mixes with the plasma upstream ofwafer 115. Various configurations and geometries of the plasma source101 and induction coils 115 may be used. For example, induction coils115 may loop around the plasma source 101 in an interlaced pattern. Inanother example, the plasma source 101 may be shaped as a dome insteadof a cylinder.

Suitable plasma apparatuses include the Gamma 2100, 2130 I²CP(Interlaced Inductively Coupled Plasma), G400, and GxT offered byNovellus Systems, Inc. of San Jose, Calif. Other apparatuses include theFusion line from Axcelis Technologies Inc. of Rockville, Md., the TERA21from PSK Tech Inc. in Korea, and the Aspen tool from Mattson TechnologyInc, in Fremont, Calif.

FIG. 2A to 2D depicts various stages of semiconductor fabrication beforeand after ion implantation and stripping operations. FIG. 2A shows asemiconductor substrate 201 coated with photoresist material 203. Thesubstrate 201 may include one or more layers of deposited film, e.g.,oxide film, silicide contact, and/or polysilicon film, or may be a baresilicon substrate, including for example a silicon-on-insulator typesubstrate. Initially, the photoresist material coats the entiresubstrate surface. The photoresist is then exposed to patternedradiation generated through a mask and developed to remove a portion ofthe material, e.g., the opening 204 shown in FIG. 2A between theremaining photoresist materials 203.

The substrate is then exposed to an ion implant process. During ionimplant, the surface of the work piece or wafer is implanted with dopantions. The process may be, for example, a plasma-immersion ionimplantation (PIII) or ion beam implantation. The ions bombard thesubstrate surface, including the exposed silicon layer 201 and thephotoresist 203. With high energy ion implantation, small amounts of theunderlying material 207 may be sputtered to the photoresist sidewalls.See FIG. 2B. This material may include some of the implant species,other material in the plasma or ion beam, and by-products of theimplantation. They include silicon, aluminum, carbon, fluorine,titanium, other contact materials such as cobalt, and oxygen in bothelemental and compound form. The actual species depend on thecomposition of the substrate before ion implant, the photoresist, andthe implanted species.

At the exposed silicon layer 201, a doped region 209 is created. The ionenergy or intensity of the bombardment determines the depth or thicknessof the doped region. The density of the ion flux determines the extentof doping.

The ions also impregnate the photoresist surface creating a crust layer205. The crust layer 205 may be carbonized and highly cross-linkedpolymer chains. The crust is usually depleted of hydrogen andimpregnated with the implant species. The crust layer 205 is denser thanthe bulk resist layer 203. The relative density depends on the ion fluxwhile the thickness of the crust layer depends on the ion energy.

This crust layer 205 is harder to strip than the bulk photoresist 203below. Removal rates of the crust layer may be 50% or 75% slower thanthe underlying bulk photoresist. The bulk photoresist containsrelatively high levels of chemically bonded nitrogen and some of itsoriginal casting solvent. At elevated wafer temperature, e.g., above 150to above 200° C., the bulk resist can outgas and expand relative to thecrust layer. The entire photoresist can then “pop” as the underlyingbulk photoresist builds up pressure under the crust. Photoresist poppingis a source of particles and process defects because the residues areespecially hard to clean from the wafer surface and chamber internalparts. With high-dose ion implantation, the density difference betweenthe crust and underlying bulk photoresist layer is even higher. Thecrust may also be thicker.

FIG. 2C shows the substrate after a strip that fails to completelyremove the photo resist 205 and the sidewall sputter residue 207. Thesidewall sputter residue 207 may include particles that do not form avolatile compound under conventional strip chemistries. These particlesmay remain after a conventional strip operation. The residue may alsoinclude oxides of implanted species formed with the reactive oxygen usedin the conventional strip chemistry, such as boron oxide and arsenicoxide. Portions of the crust 205 may also remain on the substrate. Crustsidewalls and corners at the bottom of photoresist vias may be hard tostrip because of geometries.

These residue particles may be removed by overstripping in some cases,using fluorinated chemistry, or wet cleaning the wafer. Overstripping inconventional oxygen chemistry has been found to cause unwanted siliconoxidation but still not remove boron oxide and arsenic oxide residues ifpresent. Using fluorinated compounds in plasmas generated in accordancewith this invention produces fluorine radicals that can form volatileboron fluoride and arsenic fluoride. This helps remove residues but mayunfortunately also etch underlying silicon and silicon oxide from thesubstrate. Use of the particular strip fluorinated chemistries inaccordance with embodiments of this invention mitigates this problem.

Silicon loss is a function of resist thickness, crust thickness, andpercent overstrip. Longer and more aggressive stripping to removethicker resist can also remove more silicon. For resist with thickercrust, the difference between the crust layer and bulk resist layer iseven more pronounced. The thicker crust sidewalls and corners are evenharder to strip. Thus, strip processes designed to remove thick crustalso tends to remove more silicon. Overstrip may be used to addressresist uniformity and geometries in addition to residue removal.Overstrip is a continuation of the strip process past the pointnominally required to remove all photoresist. If the photoresist istotally removed in some areas of the wafer but not others, continuationof the strip process would cause additional material, typically siliconand silicon oxide, to be removed from areas that are already stripped.Typical overstrip is about 100%.

FIG. 2D shows the substrate after all the residues has been removed.Preferably, the residue is removed without additional silicon loss oroxidation and with minimum delay. Even more preferably, the stripprocess leaves no residue and thus reduces the number of process steps.

The disclosed process and apparatus of the present invention use ahydrogen-based plasma chemistry with a weak oxidizing agent and afluorine-containing gas to achieve a substantially residue free stripprocess with minimal silicon loss. The silicon loss is believed to bereduced because fluorine radicals in the plasma combine with hydrogen inthe process gas to form hydrogen fluoride (HF) instead of remaining asfluorine radicals and etching underlying silicon. A combination ofcarbon dioxide and carbon tetrafluoride in the plasma has beendemonstrated to strip away the post high dose implant photoresistleaving the substrate residue free or substantially residue free basedon a SEM inspection or a defect inspection tool such as one fromKLA-Tencor of Milpitas, Calif. This is accomplished with minimaloverstrip (e.g., less than about 100% overstrip). According to variousembodiments, a substantially residue free condition is indicated by lessthan about 3% inspected die having polymer defects as detected by adefect inspection tool.

An acceptable minimum silicon loss may be about 3 angstroms or less,preferably less than about 1 angstrom. Device requirements drive thisminimum silicon loss regardless of resist thickness and other factorsthat may affect silicon loss. To reduce measurement errors, the siliconloss is typically measured by processing a wafer through the same stripprocess a number of times, for example, 5 times before measuring thesilicon loss on the device structure using an electronic microscope,e.g., transmission electronic microscope. The average silicon loss thusobtained is used to compare various processes.

Process Parameters

Upstream Inlet Gas

A hydrogen-containing gas, typically comprising elemental hydrogen, isintroduced to the plasma source. Typically the gas introduced to theplasma source contains the chemically active species that will beionized in the plasma source to form a plasma. The gas introduced to theplasma source includes a fluorine containing gas such as carbontetrafluoride, other fluorocarbons including C₂F₆ andhydrofluorocarbons, elemental fluorine, nitrogen trifluoride, sulfurhexafluoride. In certain embodiments, the fluorine containing gas iscarbon tetrafluoride. In certain specific embodiments, the gasintroduced to the plasma source comprises between about 0.1% to about 3%carbon tetrafluoride by volume. The gas introduced to the plasma sourcemay include a weak oxidizing agent such as carbon dioxide, carbonmonoxide, nitrogen dioxide, nitrogen oxide and/or water. In certainembodiments, the weak oxidizing agent is carbon dioxide.

According to various embodiments, the inlet gas may include betweenabout 1 and 99 volume percent, about 80 and 99.9 volume percent, orabout 95 volume percent molecular hydrogen, between about 0 and 99volume percent or 0 and 10 volume percent weak oxidizing agent, andbetween about 0.1 and 10 volume percent fluorine containing compound(s).In certain embodiments, the inlet gas may include between about 95 to 99volume percent molecular hydrogen, between about 0.1 and 3 volumepercent weak oxidizing agent, and between about 0.1 and 1 volume percentfluorine containing compound(s). In specific embodiments, the gasintroduced to the plasma source comprises about 95% to 99% elementalhydrogen, about 1-3% carbon dioxide, and about 1% or less carbontetrafluoride, all by volume.

The gas introduced to the plasma source may be premixed, partially mixedor unmixed. Individual gas sources may flow into a mixing plenum beforebeing introduced to the plasma source. In other embodiments, thedifferent gases may separately enter the plasma source. The gasintroduced to the plasma source may have different compositions whenused in different reaction stations of a multistation chamber. Forexample in a 6-station chamber, station 1 or station 6 may employprocess gases with relatively higher amounts of fluorine containing gasto remove the crust or the residue, respectively. One or more of theother stations may employ process gases with little or no fluorinecontaining gas. Process gases with no carbon dioxide or weak oxidizingagents may also be used.

Methods of stripping photoresist and etch materials using hydrogen-basedplasmas with weak oxidizing agents are disclosed in U.S. Pat. No.7,288,484, which is hereby incorporated by reference in its entirety andfor all purposes.

Plasma Generation

Various types of plasma sources may be used in accordance with theinvention, including RF, DC, and microwave based plasma sources. In apreferred embodiment, a downstream RF plasma source is used. Typically,the RF plasma power for a 300 mm wafer ranges between about 300 Watts toabout 10 Kilowatts. In some embodiments, the RF plasma power is betweenabout 1000 Watts and 2000 Watts.

Inert Gas

Various inert gases may be used in the stripping process. As explained,these gases are introduced downstream of the plasma source and upstreamof the showerhead for mixing with the plasma. In a certain embodiments,the inert gas is argon or helium. In a specific embodiment, the inertgas is argon. However, any inert gas, including nitrogen and helium, maybe used. In certain embodiments, the inert gas flow rate is betweenabout 0.15 and 10.0 times the hydrogen flow rate. In certain specificembodiments, the inert gas flow rate is between about 1 and 3 times orabout 2 times the hydrogen flow rate.

Inert Gas Inlet

The inert gas inlet may be any one of various types of gas inlets andmay include multiple ports or jets to facilitate mixing with the plasma.The angle of the inlet jets may also optimized to maximize mixing. Inone embodiment, there are at least four inert gas inlet jets. In anotherembodiment, there are sixteen inlet jets. In certain specificembodiments the angle of the inlet jets, as measured from the bottom ofthe plasma source, is zero degrees so that the inert gas is injectedperpendicular to the direction of flow of the plasma entering theshowerhead assembly (or the process chamber if there is no showerheadassembly) from the plasma source. An angle of zero degrees alsocorresponds to a direction parallel to the face of the work piece. Ofcourse, other inlet angles may be employed, although in manyembodiments, the angles are generally parallel to the work piece face.

Showerhead Assembly

According to various embodiments of the present invention the plasma gasis distributed to the work surface via a showerhead assembly. Theshowerhead assembly may be grounded or have an applied voltage toattract some charge species while not affecting the flow of neutralspecies to the wafer, e.g., 0-1000 watt bias. Many of the electricallycharged species in the plasma recombine at the showerhead. The assemblyincludes the showerhead itself which may be a metal plate having holesto direct the plasma and inert gas mixture into the reaction chamber.The showerhead redistributes the active hydrogen from the plasma sourceover a larger area, allowing a smaller plasma source to be used. Thenumber and arrangement of the showerhead holes may be set to optimizestrip rate and strip rate uniformity. If the plasma source is centrallylocated over the wafer, the showerhead holes are preferably smaller andfewer in the center of the showerhead in order to push the active gasestoward the outer regions. The showerhead may have at least 100 holes.Suitable showerhead include the Gamma xPR showerhead or the GxT drop-inshowerhead available from Novellus Systems, Inc. of San Jose, Calif.

In embodiments in which there is no showerhead assembly, the plasma andinert gas mixture enters the process chamber directly.

Process Chamber

The process chamber may be any suitable reaction chamber for the stripoperation being performed. It may be one chamber of a multi-chamberedapparatus or it may simply be a single chamber apparatus. The chambermay also include multiple stations where different wafers are processedsimultaneously. The process chamber may be the same chamber where theimplant, etch, or other resist-mediated process takes place. In otherembodiments, a separate chamber is reserved for the strip. Processchamber pressure may range from about 300 mTorr to 2 Torr. In certainembodiments, the pressure ranges from about 0.9 Torr to 1.1 Torr.

The process chamber includes one or more processing stations on whichstrip operations are performed. In certain embodiments, the one or moreprocessing stations includes a preheat station, at least one stripstation, and an over-ash station. Various features of the processchamber and the process station are disclosed in FIG. 1 and associatedtext. The wafer support is configured to support the wafer duringprocessing. The wafer support may also transfer heat to and from thewafer during processing to adjust the wafer temperature as necessary. Incertain embodiments, the wafer is supported on a plurality of minimumcontacts and does not physically contact the wafer support surfaceplane. A spindle picks up the wafer and transfers the wafer from onestation to another.

Suitable plasma chambers and systems include the Gamma 2100, 2130 I²CP(Interlaced Inductively Coupled Plasma), G400, and GxT offered byNovellus Systems, Inc. of San Jose, Calif. Other systems include theFusion line from Axcelis Technologies Inc. of Rockville, Md., TERA21from PSK Tech Inc. in Korea, and the Aspen from Mattson Technology Inc.in Fremont, Calif. Additionally, various strip chambers may beconfigured onto cluster tools. For example, a strip chamber may be addedto a Centura cluster tool available from Applied Materials of SantaClara, Calif.

Work Piece

In preferred embodiments, the work piece used in accordance with themethods and apparatus of the invention is a semiconductor wafer. Anysize wafer may be used. Most modern wafer fabrication facilities useeither 200 mm or 300 mm wafers. As disclosed above, the process andapparatus disclosed herein strips photoresist after a processingoperation such as etching, ion implant, or deposition. The presentinvention is suitable for wafers having very small features or criticaldimensions, e.g., sub 100 nm, at 65 nm, or at or less than 45 nm. Thelow silicon loss feature of the HDIS as disclosed is particularlysuitable for very shallow junctions of advanced logic devices. Thepresent invention is also specifically suitable for wafers undergoingfront end of the line (FEOL) ion implantation, especially high-dose ionimplantation.

The plasma-activated species reacts with the photoresist and sputterresidue on the wafer. At the wafer, the reactive gas may include anumber of plasma activated species, the inert gas, radicals, chargedspecies, and gas by-products. The volume concentration of varioushydrogen species may be about 20-80% of the gas at the wafer. The volumeconcentration of various fluorine species may be 0.01% to about 2% orless than 1%. The volume concentration of various species from the weakoxidizing agent may be 0.05 to about 5% or about 1.2%. These species mayinclude H₂ ^(*), H₂ ⁺, H⁺, H^(*), e⁻, OH, O^(*), CO, CO₂, H₂O, HF,F^(*), F⁻, CF, CF₂, and CF₃.

Process conditions may vary depending upon the wafer size. In someembodiments of the invention, it is desired to keep the work piece at aparticular temperature during the application of plasmas to its surface.Wafer temperatures can range between about 110 degrees and about 500degrees Celsius. To reduce the likelihood of photoresist poppingdescribed above, wafer temperature is preferably increased slowly untilenough crust has been removed and photoresist popping ceases to be aconcern. Initial station temperature may be about 110 degrees to about200 degrees Celsius, for example, about 180 degrees Celsius. Laterstations can use higher temperatures such as 285 degrees Celsius andabout 350 degrees Celsius successfully with good strip rates.

Process Flow

FIG. 3 is a process flow diagram showing various operations inaccordance with certain embodiments of the present invention. A wafer ispositioned in a reaction chamber on a wafer support. At operation 301, ahydrogen-containing gas is introduced to a plasma source. A plasma isgenerated from the gas in operation 303. As more gas is added to theplasma source, the plasma flows downstream and mixes with an inert gasintroduced at operation 305. Some of the charged species in the plasmamay combine to form neutral, albeit activated, species. Together, theactivated species and the inert gas flow through a showerhead face plateand react with photoresist on a wafer surface at operation 307. Thereaction results in volatile by-products that are removed by theprocessing area with a vacuum pump at operation 309. The process may berepeated one or more times using different process parameters. Forexample, the wafer may be heated or cooled during iterations of theprocess. In another example, different initial hydrogen-containing gasand inert gas composition and flow rates may be used. Preferably, atleast one of the iterations involves hydrogen-based gas includingelemental hydrogen, carbon dioxide, and carbon tetrafluoride. One ormore of the iterations may involve hydrogen-containing gas that does notinclude carbon dioxide or carbon tetrafluoride.

According to various embodiments, the various iterations may be designedto target different portions of the photoresist, for example, the posthigh-dose implant resist having a crust and bulk resist regionsdiscussed above. The first strip iteration in a first strip station maybe designed to strip the crust layer. The first strip iteration mayinvolve generating a plasma using elemental hydrogen, carbon dioxide (oranother weak oxidizing agent), and with or without carbon tetrafluoride(or another fluorine containing gas) to specifically strip the crustlayer. When the crust layer is sufficiently thin or completely removed,a second strip iteration may strip the bulk resist along with theresidue and the remaining crust layer, often at a higher wafertemperature. The second strip process may be performed in a differentprocessing station from the first strip process. The second stripprocess may employ plasma generated without the weak oxidizing agent orthe fluorine containing gas, or both. After the bulk resist is removed,yet another strip process employing different gas compositions may bedesigned to strip the residue, if any. This residue stripping processmay employ a fluorine containing gas to remove any oxidized implantspecies. The strip iteration described above may be performed in anyorder or frequency depending on the number of processing stations andthe composition of the photoresist to be stripped. One skilled in theart would be able to tailor the concepts discussed herein to stripthicker or thinner crust having lower or higher resistance to the stripchemistry. Further, the concepts discussed herein is applicable to othersituations when more than one layer of photoresist having differentproperties are stripped by different using strip chemistries to targetdifferent resist layers.

EXAMPLE 1

In this example, the impacts of carbon dioxide and carbon tetrafluorideon residue were investigated. 300 mm sized wafers were patterned with 45nm structures and ion implanted with an LDD (lightly doped drain) in theP+ region. The resulting post high dose implant resist was about 2000angstroms thick with a crust about 630 angstroms thick.

The wafers were stripped in a strip chamber having 5 plasma stations.The plasma was generated with RF power at 2000 Watts. The wafers wereexposed to the plasma-activated reactive gas for about 20 seconds ateach station for a total of 97 seconds. The wafer support temperaturewas 350 degrees Celsius. Chamber pressure was 900 mTorr. Hydrogen flowrate was 6 slm (standard liters per minute), and downstream argon flowrate was 14 slm. Carbon dioxide flow rate was varied between 0 and 150sccm. Carbon tetrafluoride flow rate was varied between 20 and 40 sccm.Note that these flow rates are total flow rates for the entire chamberwith 5 plasma stations. Each station receives about ⅕ of the total flowrate.

SEM photos of the wafers before and after stripping with variousplasma-activated reactive gases are depicted as FIGS. 4A to 4D. FIG. 4Adepicts a small portion of the wafer before stripping. Structure 401 isthe post high-dose implant resist. Pads 405 include structures 403thereon where the photoresist was removed during the patterning process.Thus the HDIS process removes structure 401.

In the first wafer as depicted in FIG. 4B, 20 sccms of carbontetrafluoride and 150 sccms of carbon dioxide were added to the hydrogento form the plasma. Worm like residues 407 remained after the process.In the second wafer as depicted in FIG. 4C, 40 sccms of carbontetrafluoride and 150 sccms of carbon dioxide were added to the hydrogento form the plasma. The strip resulted in no residue, as shown in FIG.4C. In yet another wafer as depicted in FIG. 4D, 40 sccms of carbontetrafluoride and no carbon dioxide was added to the hydrogen to formthe plasma. Worm like formation of residue 407 was again observed. Thisresult shows that the addition of carbon dioxide with carbontetrafluoride can yield residue free film in hydrogen-based HDIS.

EXAMPLE 2

In this example, the effects of carbon dioxide flow rate and carbontetrafluoride flow rate on silicon loss were independently examined.Silicon loss for HDIS under the same process conditions as that ofExample 1 was measured for carbon dioxide flow rates of 0, 50, 100, and150 sccm while the carbon tetrafluoride flow rate was held constant at40 sccm. The results are plotted in FIG. 5A. Silicon loss is lowest atcarbon dioxide flow rate of 150 sccm and highest when no carbon dioxideis added. This result shows that some carbon dioxide in the plasmareduces silicon loss.

Silicon loss for HDIS under the same process conditions as that ofExample 1 was also measured for carbon tetrafluoride flow rates of 0,40, 60, 80, and 100 sccm while carbon dioxide flow rate was heldconstant at 150 sccm. The results are plotted in FIG. 5B. The siliconloss appears to peak at carbon tetrafluoride flow rates of 60 to 80sccm.

These results show that silicon loss is affected by flow rates of carbondioxide and carbon tetrafluoride. For a particular film, one skilled inthe art would be able to design an HDIS process that minimizes siliconloss and leaves the film residue free.

EXAMPLE 3

In another example, the effect of using different gas compositions atdifferent stations was investigated relative to silicon loss and stripresidue. The process conditions were the same as that of Example 1,except that the wafer support temperature is 250 degrees Celsius. In afirst recipe, carbon tetrafluoride was employed in all stations at atotal flow rate of 40 sccm. In a second recipe, carbon tetrafluoride wasdelivered only to RF stations 1 and 2 at a total flow rate of 20 sccm(10 sccm per station). Carbon dioxide flow rate was held constant at 150sccm.

In both cases residue free substrates were obtained after HDIS process.The average silicon losses were 8.1 Å per cycle in the first recipe and6.7 Å in the second recipe, a reduction of about 17%. A cycle is acomplete pass through the tool, including processing at all stations.This result shows that a sequential stripping process using differentgas compositions can reduce silicon loss while maintaining residue freesubstrates.

EXAMPLE 4

In this example, the effect of short process time per station and lowercarbon tetrafluoride flow rate was investigated. In the first recipe, nocarbon tetrafluoride was added and process time was 20 seconds perstation. In the second recipe, carbon tetrafluoride was added at 10 sccmwith strip process time of 10 seconds per station. In both recipes thewafer support temperature was 285 degrees Celsius.

In the first recipe, without the carbon tetrafluoride in the plasma,residue was found after the strip. The average silicon loss per cyclewas 1.93 Å. In the second recipe, with the reduced carbon tetrafluorideflow and reduced process, the substrate was residue free with an averagesilicon loss per cycle of 3.12 Å. Although the first recipe had thelower silicon loss, the substrate after the strip was not residue free.This result shows that a reduced carbon tetrafluoride flow rate may beused with shorter process time to yield a residue free substrate.

EXAMPLE 5

In this example, carbon tetrafluoride was introduced into the plasmasource at different stations. In the first recipe, 5 sccm of carbontetrafluoride was introduced in RF station 1. In the second recipe, 5sccm of carbon tetrafluoride was introduced in RF station 3. Siliconloss was measured after the first cycle and after the fifth cycle andaveraged. The other process parameters are the same as that of example1.

The first recipe yielded a residue free substrate. Silicon loss afterthe first cycle with carbon tetrafluoride was 14.4 Å. Silicon loss afterthe fifth cycle was 18.6 Å. The average silicon loss per cycle decreasedfrom 14.4 Å to 3.7 Å.

Small amounts of residue were observed on the substrate after HDIS withthe second recipe. Silicon loss after the first cycle was 6.9 Å, lessthan that of the first recipe. Silicon loss after the fifth cycle was10.3 Å. The average silicon loss per cycle decreased from 6.9 Å to 2.1Å.

This result shows that silicon loss using this chemistry is aself-limiting reaction where majority of silicon loss takes place in thefirst cycle. Additional processing does not remove much more silicon.This is an advantage over the conventional oxygen with fluorinestripping chemistry where total silicon loss is proportional toprocessing time. In cases where overstripping is necessary, e.g., whenphotoresist thickness is not uniform, an oxygen chemistry would causemore silicon loss than the hydrogen chemistry as disclosed.

This result also shows that the first cycle silicon loss is reduced ifcarbon tetrafluoride is not used. One skilled in the art may be able todelay the introduction of carbon tetrafluoride to reduce the totalsilicon loss.

Note that experimental results for these specific examples are shown toclarify and illustrate the effectiveness of methods of the invention andare not meant to limit the invention to any particular embodiments.

1. A method of removing material from a work piece surface in a reactionchamber, the method comprising: introducing a gas comprising elementalhydrogen, a weak oxidizing agent, and a fluorine containing gas into aplasma source; generating a plasma from the gas introduced into theplasma source; and introducing an inert gas downstream of the plasmasource and upstream of the work piece, wherein the gas comprisingelemental hydrogen, the weak oxidizing agent, and the fluorinecontaining gas flows, together with the inert gas, to the work piece andreacts with the material from the work piece.
 2. The method of claim 1,wherein the inert gas is selected from a group consisting of argon,helium, nitrogen, and combination thereof.
 3. The method of claim 1,wherein introducing the inert gas comprises introducing the gas upstreamof a showerhead in the reaction chamber.
 4. The method of claim 3,wherein electrically charged species in the plasma are discharged whenthey contact the showerhead.
 5. The method of claim 1, wherein the weakoxidizing agent is selected from a group consisting of carbon dioxide,carbon monoxide, nitrogen dioxide, nitrogen oxide, water, hydrogenperoxide and combinations thereof.
 6. The method of claim 1, wherein theweak oxidizing agent is carbon dioxide.
 7. The method of claim 1,wherein the gas introduced into the plasma source comprises betweenabout 0.1% to about 10% by volume of the weak oxidizing agent.
 8. Themethod of claim 1, wherein the fluorine containing gas is selected froma group consisting of carbon tetrafluoride, elemental fluorine, nitrogentrifluoride, sulfur hexafluoride, fluorocarbons, hydrofluorocarbons andcombinations thereof.
 9. The method of claim 1, wherein the fluorinecontaining gas is carbon tetrafluoride.
 10. The method of claim 1,wherein the fluorine containing gas is CF₄, C₂F₆, CHF₃, CH₂F₂, C₃F₈, orNF₃.
 11. The method of claim 1, wherein the gas introduced into theplasma source comprises between about 0.1% to about 3% by volume of thefluorine containing gas.
 12. The method of claim 1, wherein the materialremoved from the work piece surface comprises a high-dose implantedresist.
 13. The method of claim 1, wherein the volumetric flow rate ofthe inert gas is between about 0.15 and 10 times volumetric flow rate ofthe elemental hydrogen.
 14. The method of claim 1, wherein thevolumetric flow rate of the inert gas is at least about 2 timesvolumetric flow rate of the elemental hydrogen.
 15. The method of claim1, wherein the gas introduced into the plasma source is premixed. 16.The method of claim 1, wherein the work piece is a 300 mm wafer and theplasma is generated by RF power ranging between about 300 Watts andabout 10 Kilowatts.
 17. The method of claim 1, wherein the temperatureof the work piece is between about 160 degrees and about 400 degreesCelsius when contacted by the gas comprising elemental hydrogen, theweak oxidizing agent, and the fluorine containing gas.
 18. The method ofclaim 1, wherein the pressure in the process chamber is between about300 mTorr and about 2 Torr.
 19. The method of claim 1, wherein ahigh-dose implant resist is removed from the work piece surface at arate that is at least about 100 nm/min and silicon is removed from thework piece surface at an overall rate of no greater than about 4 nm/min.20. The method of claim 12, wherein the work piece is substantiallyresidue free of the high-dose implanted resist after removal and whereinless than about 3 angstroms silicon is lost from an underlying siliconlayer.
 21. A method of removing high-dose implanted resist from a workpiece surface in a reaction chamber, the method comprising: removing afirst portion of the material comprising: introducing a first gas at afirst total flow rate comprising elemental hydrogen, a weak oxidizingagent, and a fluorine containing gas into a plasma source; generating afirst plasma from the first gas introduced into the plasma source;introducing a first inert gas downstream of the plasma source andupstream of the work piece to form a first mixture; reacting a firstportion of the material from the work piece with the first mixture;removing a second portion of the material comprising: introducing asecond gas at a second total flow rate comprising hydrogen and a weakoxidizing agent into a plasma source, said second gas composition isdifferent from the first gas; generating a second plasma from the secondgas introduced into the plasma source; introducing a second inert gasdownstream of the plasma source and upstream of the work piece to form asecond mixture; reacting a second portion of the material from the workpiece with the second mixture; wherein less than about 3 angstroms ofsilicon is lost from an underlying silicon layer, and wherein the workpiece is substantially residue free after the material removal.
 22. Themethod of claim 21, wherein the removing a second portion operationoccurs before the removing a first portion operation.
 23. The method ofclaim 21, wherein the removing operations of the first and/or the secondportion are repeated one or more times.
 24. The method of claim 21,wherein the removing a first portion operation and removing the secondportion operation occurs in different reaction stations in the reactionchamber.
 25. The method of claim 21, wherein the removing a firstportion operation and removing the second portion operation occurs indifferent reaction stations at different temperatures in the reactionchamber.
 26. The method of claim 21, wherein the second gas issubstantially free of any fluorine containing gas.